Previous studies have demonstrated that intraperitoneal injections of tetracycline hydrochloride to Sprague-Dawley rats can give rise to hypoplastic lesions in the enamel of any tooth that is under formation at the time of drug administration. The aim of the investigation this past year has been to define in detail the relationship of dose size, number of injections, concentraion of drug and age of the experimental animal to serum levels of the antibiotic and incidence of enamel and dentin defects. In addition to these studies experiments have been initiated aimed at establishing the effect of tetracycline on the conversion of amorphous calcium phosphate to crystalline hydroxy apatite in both an in vitro and in vivo system.